EVALUATION OF LUBRICATION AND FRICTION IN COLD FORGING USING A DOUBLE BACKWARD-EXTRUSION PROCESS

In cold-forging operations, the material flow and the quality of forged parts are functions mainly of the tool geometry, the amount of deformation or strain, and the conditions at the workpiece/tool interface (e.g., friction, relative surface velocity, surface finish and heat transfer). While many of these parameters are well known and controllable, the influence of friction is often difficult to predict and depends on a variety of factors. To reduce the amount of friction in cold forging, billets are generally coated (e.g., phosphate coating) and lubricated. In this paper a friction test, based on a double backward-extrusion process, is proposed and examined in order to obtain information on lubrication quality. In this test, the upper punch moves downwards, while the lower punch and the die are stationary. An FEM analysis, using the program DEFORM, has been conducted for different area reduction ratios and billet heights. The simulation results show reasonable agreement with the results of experimental studies performed in Germany some years ago. The reduction ratio that gives the greatest differences in extruded cup heights was selected for the test design and the influence of friction shear factors between m = 0.08 and m = 0.20 was investigated. Based on the FEM simulations, calibration curves were established. Using these calibration curves and measuring only the heights of the extruded cups and the punch stroke in experiments enables the quantification of the friction factor and the evaluation of the lubrication conditions under production conditions.